1. Field of the Invention
The present invention relates to a method of manufacturing a ceramic paste and a ceramic multi-layer wiring substrate having an internal wiring utilizing the same. More particularly, the invention relates to a method of manufacturing a ceramic multi-layer wiring substrate formed by a plurality of ceramic layers stacked one on top of another and having an internal wiring, in which a ceramic filling layer including the same ceramic component as that in the ceramic layers is formed in a region having no internal wiring formed therein between the ceramic layers stacked so that the internal wiring is sandwiched therebetween. More specifically, the invention relates to a method of manufacturing a ceramic paste which is preferable for fabricating a ceramic green sheet having substantially no irregularity and thereby preventing a layer bonding failure and warping deformation of a ceramic multi-layer wiring substrate more effectively, and a ceramic multi-layer wiring substrate using the ceramic paste.
2. Description of the Related Art
A ceramic multi-layer wiring substrate according to the related art has a structure in which a metalized wiring layer is disposed on a surface or inside of an insulated substrate formed by stacking a plurality of insulation layers. A typical example of ceramic multi-layer wiring substrates having such a structure is semiconductor device containing packages which contain semiconductor devices such as LSIs. Semiconductor device containing packages of this type having insulation layers made of ceramics such as alumina are frequently used. Further, products which have recently been put into practical use include products utilizing an insulated substrate formed by stacking insulation layers constituted by a glass-ceramic sintered body that can be fired simultaneously with a copper-metalized material.
Such a ceramic multi-layer wiring substrate is manufactured by adding an appropriate organic binder to powder of ceramic materials mixed in a predetermined ratio, dispersing the resultant powder in an organic solvent to prepare slurry, and molding the slurry into a ceramic green sheet (hereinafter referred to as a green sheet) having a predetermined thickness using a casting process well-known in the related art such as a doctor blade process or lip coater process.
Next, a metalized wiring layer is provided by printing metal paste, which is obtained by adding and mixing an organic binder, a solvent, and a plasticizer in appropriate metal powder, in a predetermined pattern on the green sheet using the well-known screen printing process. Simultaneously through holes are formed in the resultant layer using a micro-drill or laser, and the through holes are filled with the metal paste to form through conductors (via conductors or via holes).
Thereafter, a plurality of such green sheets are stacked using an appropriate bonding liquid, and the resultant stack of green sheets is fired under predetermined conditions to obtain a ceramic multi-layer wiring substrate.
Related art is disclosed in Japanese Unexamined Patent Publications JP-A 5-3273 (1993) and JP-A 11-97272 (1999).
The ceramic multi-layer wiring substrates (hereinafter also simply referred to as substrates) according to the related art as described above, tend to be made smaller and thinner associated with the increase and advance of functions of such substrates. Further, wiring patterns and via holes formed on a substrate are becoming finer, which has resulted in demands for improvement in accuracy of stacking. Particularly, under the circumstance in which ceramic multi-layer wiring substrates are becoming thinner, gaps in such a substrate generated during formation of an internal wiring due to a height difference attributable to the thickness of the internal wiring are difficult to eliminate completely during layer stacking by utilizing only plastic deformation of a green sheet whose thickness is small. As a result, there arises a problem that a layer bonding failure or so-called delamination and warping deformation can occur in such a product after it is fired.
As means for solving this problem, a screen printing process is performed to form a ceramic filling layer including a same ceramic component as that in ceramic layers in a region between the ceramic layers which are stacked so that an internal wiring is sandwiched therebetween, the region having no internal wiring formed therein.
However, a green sheet having a ceramic filling layer obtained by the screen printing process has a problem in that the layer pattern can bulge at edges thereof due to surface tension when the screen is separated from the same, and a phenomenon also occurs in which the pattern bulges in a substantially semicircular or semi-elliptic shape when the pattern is constituted by very fine lines having a great thickness. It is therefore difficult to eliminate irregularities on the surface of the sheet completely, and it is required to add a step for pressing the sheet with a surface-leveling press to level the same.
A cellulose type binder which generally has high printability is often used as a binder for a ceramic paste used in a screen printing process. However, since a ceramic paste utilizing a cellulose type binder has low fluidity, it is difficult to level the paste sufficiently at the pressure leveling step. Under the circumstance, it is a general practice to add an acryl type binder to a cellulose type binder in ceramic paste for the purpose of providing it with plasticity and to improve fluidity of the binder by adding it with a plasticizer such as phthalic ester and a solvent having a high boiling point in a somewhat large amount.
However, when the balance of viscoelastic properties of ceramic pastes is not properly controlled, it is difficult to level a ceramic paste without deforming an internal wiring pattern adjacent to the ceramic paste if the pressure leveling is performed under a temperature condition that is equal to or higher than the glass-transition temperature of an acryl type binder added to the ceramic paste. Specifically, when loss factor tan δ=(loss modulus)/(storage modulus) of the ceramic paste in a dry condition at a temperature near the heating temperature of the pressure leveling step is smaller than loss factor tan δ of a conductor paste layer to be an internal wiring in a dry condition, a stress is applied from the ceramic paste whose behavior is dominantly elastic to the internal wiring layer whose behavior is dominantly viscous. As a result, there is a high possibility of the problem that the internal wiring is significantly deformed.
It is therefore difficult to protect the adjacent internal wiring from deformation during pressing only by increasing the elastic modulus of the ceramic paste used for the internal wiring or, in other words, by providing means for reducing the loss factor tan δ of the ceramic paste used for the internal wiring in a dry condition to form a more rigid ceramic paste layer whose behavior is dominantly elastic.
When an acryl type binder is used without adjusting its molecular weight properly, a problem is frequently created by spinnability that is a shortcoming of the use of an acryl type binder in screen printing.
When a plasticizer and solvent are excessively used, a ceramic paste tends to have high viscosity attributable to insufficient drying even if a drying step is performed after screen printing. Therefore, a resultant printed material may remain viscous or may have low strength, which results in a problem in that the printed material can be subjected to cohesive breakage or interfacial breakage and transferred to a surface of the press during the pressure leveling step.